Network Address Translation [NAT] is a process that allows multiple devices within a private network
to access the internet using a single public IP address.
NAT is essential for conserving public IP addresses and enhancing privacy by hiding the internal IP addresses of network devices.
It translates private IP addresses to public IP addresses and vice versa, typically operating on a router or firewall.
NAT is a mechanism in which one or more private [local] IP addresses are translated into one or more public [global] IP addresses.
It also handles the translation of port numbers to facilitate internet access for internal hosts.
By masking the internal IP and port numbers with a public IP and alternate port numbers,
NAT ensures seamless communication between the internal network and external systems.
NAT keeps track of these translations using a NAT table, which logs the original and translated IP addresses and port numbers.
NAT typically operates on a router that serves as a gateway between the internal [private] network and the external [public] network.
NAT is generally implemented on a border router,
a router that has one interface connected to the internal [local] network and another to the external [global] network.
The process works as follows:
When a packet leaves the internal network, NAT translates the local [private] IP address to a global [public] IP address.
When a packet returns to the internal network, NAT converts the global [public] IP address back to the local [private] IP address.
If the NAT device runs out of public IP addresses from the configured pool, the packets will be dropped,
and an ICMP [Internet Control Message Protocol] message is sent, indicating that the host is unreachable.
Port numbers are masked by NAT to avoid conflicts when multiple internal hosts communicate with the same external destination using the same port.
For instance, if two internal hosts [A and B] request access to the same external server on port 1000, NAT must distinguish between their connections.
Simply translating their IP addresses wouldn't be enough, as both requests would appear to come from the same public IP.
To solve this, NAT modifies the source port numbers and logs them in the NAT table, ensuring the correct responses are routed back to the right hosts.
Inside Local Address - The private IP address assigned to a host within the internal network.
This address is typically not visible from the outside.
Inside Global Address - The public IP address assigned to represent the internal host to the external network.
Outside Local Address - The private IP address of an external host, used within the internal network after translation.
Outside Global Address - The public IP address of an external host, used before translation occurs.
Static NAT - A one-to-one mapping is created between a private [unregistered] IP address and a public [registered] IP address.
This method is often used for services like web hosting, where a specific internal server needs to be accessible from the outside.
However, static NAT is not suitable for large-scale networks since each internal device would require a unique public IP address,
which can be costly and impractical.
Dynamic NAT - Private IP addresses are mapped to public IP addresses from a predefined pool. The translation occurs only if
a public IP is available in the pool.
If the pool runs out of available public IP addresses, additional requests will be dropped.
This method is more scalable than static NAT, but still limited by the size of the public IP address pool.
Port Address Translation [PAT] / NAT Overload - Allows multiple private IP addresses to share a single public IP address by distinguishing traffic based on port numbers.
This is the most commonly used form of NAT because it allows thousands of devices to access the internet using just one public IP address, making it highly cost-effective.
PAT modifies both IP addresses and port numbers, keeping track of each session in the NAT table.
Advantages of NAT:
IP Address Conservation - NAT reduces the number of public IP addresses needed by allowing multiple devices to share a single public address.
Enhanced Privacy - By masking internal IP addresses, NAT hides the details of internal devices from the external network.
Flexibility in Network Addressing - NAT eliminates the need for renumbering internal IP addresses when the network grows or changes.
Disadvantage of NAT:
Increased Latency - Translating IP addresses and port numbers can introduce delays in packet forwarding, especially in high-traffic networks.
Incompatibility with Certain Applications - Some applications, particularly those that embed IP address information within their payloads
[e.g., certain VoIP or VPN services], may not function properly with NAT.
Complication of Tunneling Protocols - NAT can interfere with tunneling protocols such as IPsec, which require direct IP address visibility.
Layer Violation - NAT operates at the network layer, but it manipulates transport-layer data [i.e., port numbers], which can lead to complexities in troubleshooting and performance optimization.
NAT plays a crucial role in managing the depletion of IPv4 addresses while enhancing network security by obscuring internal IP addresses.
Its cost-effectiveness, particularly with PAT, makes it indispensable for networks requiring internet connectivity for a large number of devices.
Despite its limitations, such as increased latency and potential conflicts with specific applications, NAT remains a widely used solution, particularly during the transition from IPv4 to IPv6.